As a display device, a liquid crystal display including an array substrate where a switching element and a pixel electrode are formed, a counter-substrate opposed to the array substrate, and a liquid crystal layer interposed between the array substrate and the counter-substrate has been widely used.
It is known that ions resulting from impurities or ions dissolved from a sealant which attaches the array substrate to the counter-substrate are mixed in the liquid crystal layer. If the ion concentration locally increases in the liquid crystal layer, the effective voltage to be applied to the liquid crystal layer will decrease in this portion. As the effective voltage decreases, the luminance in this portion decreases, accordingly, and the decrease in the luminance will lead to a negative effect such as a dark spot on the display image.